Information reproducing method and an information reproducing drive (s)

ABSTRACT

Disclosed herewith is an information reproducing method for realizing significant expansion of disk capacity and processing signals having different minimum run lengths, as well as an optical disk drive that uses the method. To achieve the above objects, a PRML method is used. According to the method, a compensation value is added to an initial target value decided by a convolution operation of NN bits according to a bit array consisting of N bits (N&gt;NN) to obtain a new target value, which is then compared with each of reproduced signals sequentially to select a bit array in which the error between the reproduced signal and the target signal is minimized most likely, then the selected bit array is binarized.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an information reproducingmethod that uses replaceable optical disks and an informationreproducing apparatus that employs the method.

[0003] 2. Description of Related Art

[0004] The use of such optical disks as CD and DVD is now widespread anddevelopment of next generation optical disks that use a blue laser alsomakes rapid progress to meet the demand of users for increasing thecapacity of those disks more and more. And, it has become common now forone optical disk player to be provided with functions of not onlyreproducing information from CD disks or from both of CD and DVD disks,but also recording information on all of CD-R/RW, DVD-RAM, and DVD-R/RWdisks. And accordingly, the users have come now to demand thecompatibility of recording/reproducing functions among those opticaldisks manufactured in various standards to be more improved.

[0005] The PRML (Partial Response Maximum Likelihood) method, which isremarkably effective for improving the S/N ratio, is now widespread asmeans for expanding the capacity of magnetic disks. The PRML methodcompares reproduced signals with a target signal sequentially for acontinuous N time to transform each reproduced signal bit array to themost likelihood binary bit array. Conventionally, a direct slicingmethod has long been employed to reproduce information from opticaldisks. However, the conventional method has its limit in speeding up theprocessing and expanding the disk capacity. This is why the PRML methodhas come to be employed as means for reproducing information from thoseoptical disks.

[0006] In spite of such an advantage, the PRML method has beenconfronted with the following three problems that must be solved beforethe method is to be used positively as means for reproducing informationfrom optical disks.

[0007] The first problem is asymmetry occurrence. A PRML target signalis calculated by a convolution processing of a specified impulseresponse (PR class) and a bit array, so that the PRML target valuebecomes symmetrical vertically to the center value. On the other hand,each optical disk usually generates asymmetry to improve the S/N ratio.As a result, the reproduced signal level becomes asymmetrical and hardlymatches with the PRML target signal.

[0008] The second problem is an impossibility to realize significantexpansion of disk capacity. There has never been proposed any method forsuch significant expansion of disk capacity with use of the PRML methodso far. This point will be described later in detail.

[0009] The third problem is the minimum run length. The PRML basicstructure is decided uniquely by a PR class and the run length of areproduced signal. However, the minimum run length of MO disks (opticaldisks in the next generation) is 2T while that of the CD/DVD disks is3T. Consequently, to enable one optical disk drive to correspond to sucha plurality of optical disk types, the drive must also be provided witha plurality of PRML circuits.

[0010] Technical Digest of ISOM 2002 (269-271 (2002)) discloses anapplication example of the PRML method to an optical disk. In thedocument, a target signal level is changed according to thecircumstances to reproduce information from the optical disk while thedisk tilts in both radial and tangential directions. This method isexcellent, since it can solve the first problem described above(asymmetry occurrence). However, the method is still difficult to solvethe second problem (impossibility to realize significant expansion ofdisk capacity) and cannot solve the third problem (impossibility toprocess signals having different minimum run lengths in the same way).

[0011] [Non-Patent Document 1]

[0012] Technical Digest of ISOM 2002 (269-271 (2002))

[0013] As described above, the conventional PRML method is hardlyeffective to expand the capacity of optical disks significantly andcannot process signals having different minimum run lengths in the sameway.

[0014] Under such circumstances, it is an object of the presentinvention to provide an information reproducing method that can solvethe above conventional technical problems, realize significant expansionof disk capacity, and process signals having different minimum runlengths in the same way, as well as an information reproducing apparatusthat employs the method.

SUMMARY OF THE INVENTION

[0015] At first, a description will be made for the PRML method employedto expand the capacity of optical disks significantly. As describedabove, a target signal of this PRML method is decided by a selected PRclass and a reproduced signal. Therefore, the inventor et al have testedthe difference of high density recording performance among PR classes byrecording signals having different recording density values on oneoptical disk. The optical disk used for the test is formed by laminatingphase-change films on a substrate structured so as to have lands andgrooves at track pitches of 0.34 um. A DDU-1000 optical disk evaluator(PulseTech Products Corporation) was used for the test. The wavelengthof the light source was 405 nm and the NA of the objective lens was0.85. The RLL (1, 7) was used as a demodulation code and the width ofthe detection window was changed within a range of 53 nm to 80 nm. Therecording capacity of one sided CD-size disk was assumed as 35 GB forthe calculation at Tw=53 nm. The following three series were selected toexamine the difference among PR classes.

[0016] (1) (1+D)^(n) Series

[0017] This is the most basic class series consisting of PR(1, 1), PR(1,2, 1), PR(1, 3, 3, 1), .

[0018] (2) (1, 2, . . . , 2, 1) Series

[0019] This series includes the PR (1, 2, 2, 1) series that is oftenused for optical disks. The high range emphasis is less than that of theabove series (1) while it is expected to improve the S/N ratio.

[0020] (3) Impulse Response Approximation Series

[0021] A PR class is basically a approximation impulse response of areproducing head. The PR class used here is obtained by calculating animpulse of an optical head with use of an optical simulator.

[0022] Each selected PR class and the above described optical simulatorwere used to calculate idealistic reproduced signals, which were thenused to reproduce signals from the above described optical disk whilethe equalizing conditions were decided for each PR class so as tominimize the squared error between target and each reproduced signal.The number of taps of the equalizer was 11.

[0023]FIG. 2A through FIG. 4 show measurement results of thereproduction performance of the sample optical disk with respect to eachPR class series.

[0024]FIG. 2A shows a relationship between the recording capacity andthe bit error rate with respect to the (1+D)^(n) series. FIG. 2B shows atable for storing set values of bit-expression of each PR class, thenumber of effective bit arrays, the number of effective states, thenumber of independent target levels, and capacity upper limit. Thecapacity upper limit denotes a range in which the bit error rate is 10⁻⁴and under. If the number of class bits (the number of elements includedin the PR class expression) is assumed as N, the total number of bitarrays becomes 2^(N). However, because of the run length limitation, thenumber of effective bit arrays becomes a value obtained by subtractingbit arrays that have the minimum run length of 1T respectively from aset of bit arrays. The number of effective states is also obtainedsimilarly. And, because a circuit scale is proportional to the number ofeffective bit arrays to express those numbers, the number of class bitsshould preferably be as less as possible. Although the more the numberof class bits is, the more the performance improvement hits the ceilingwhen the number of class bits reaches 6 and over. The maximum capacitybecomes 31 GB when the number of class bits is 7.

[0025]FIG. 3A shows a relationship between the recording capacity andthe bit error rate with respect to the series (1, 2, . . . , 2, 1). FIG.3B shows the details of the relationship. In this series, if the numberof class bits is excessively large, the capacity is reduced. This isbecause the reproduced signal changes with time can be expressed moreaccurately when the number of class bits is large such way, the numberof independent target levels also increases at that time, thereby, thedifference between target levels with respect to two different pathsdecreases and the number of errors to occur at the time of pathselection increases. The maximum recording capacity of this seriesbecomes 31 GB when the number of class bits is 5.

[0026]FIG. 4A shows a relationship between the recording capacity andthe bit error rate with respect to the impulse response approximationseries. FIG. 4B shows the details of the relationship. The recordingcapacity of this series is also reduced when the number of class bitsincreases excessively. The maximum recording capacity of this seriesbecomes 32 GB when the number of class bits is 5.

[0027] From the results of the examination for the possible three typesof PR class series, it has been found that the performance improvementhas its limit in any complicated configuration of the optical diskdrive. This is because each reproduced signal from the optical diskcomes to have a non-linear edge shift caused by the inter-codeinterference caused by the light spot shape and the thermal interferencewhen in reproducing. And, in order to cope with such a non-linearinter-symbol interference and such an edge shift, the basic PRML methodthat decides a target value by a linear convolution processing isinsufficient, so that the non-linear component must be compensated bysome means or other. To realize a higher recording density, therefore,the following two points come to be very important.

[0028] (1) The number of class bits is prevented from increasing so asnot to increase the number of target levels.

[0029] (2) A compensation value is added to a target value that isdecided by a convolution processing according to the subject bit arrayto compensate the target value, thereby coping with each non-linearcomponent included in each reproduced signal.

[0030] To satisfy those requirements and realize significant expansionof optical disk capacity, the PRML method may be used. The PRML methodadds a compensation value to a target value decided by a convolutionoperation of NN bits according to the N-bit (N>NN) bit array to obtain anew target value, then compares the new target value with eachreproduced signal to transform a bit array selected from the N-bit bitarrays to a most likelihood binary bit array, in which the error betweenthe reproduced signal and the target value is minimized.

[0031] Next, a description will be made for a method that can processsignals having different minimum run lengths in the same way. At first,a description will be made for a case in which an RLL (1, 7) modulatedsignal (the minimum run length: 2T) used for MO disks (next generationoptical disks) is reproduced with the PR class PR(1,2,2,1). In thisconnection, because the number of class bits becomes 4, the number ofbit arrays becomes 16 (=2⁴). FIG. 6A shows target level values for those16 bit arrays. Generally, the reflection rate of the recording markshaving a digital value “1” respectively is low while the reflection rateof the recording marks having a digital value “0” is high on opticaldisks. In this case, signals are standardized so that the target levelof the bit array “0000” becomes “+1” and the target level of the bitarray “1111” becomes “−1”. Hereinafter, note that the samestandardization will apply to each description that uses a class targetvalue. In FIG. 6, if the minimum run length in a bit array is less than2T, a run length limit error occurs in the bit array. In FIG. 6A, such arun length limit error occurs in 6 bit arrays. FIG. 6B collects the bitarrays, in each of which no run length limit error occurs. Thus, PRclass PR (1, 2, 2, 1) with the minimum run length 2T can be expressed ina total of 10 bit arrays. In the prescriptive expression of the PRMLmethod, the number of states becomes 6 and the number of target levelsbecomes 7.

[0032] On the other hand, the minimum run length of CD/DVD disks is 3T.Now, a description will be made for how to reproduce signals having sucha 3T run length with the PR class PR(3, 4, 4, 3). FIG. 7A shows targetlevels for all the bit arrays. The run length limit error occurs ineight of those bit arrays. FIG. 7B shows only the effective bit arraysextracted from all the bit arrays. The PR class PR (3, 4, 4, 3) with theminimum run length 3T can be expressed in eight bit arrays. The numberof states becomes 6 and the number of target levels becomes 5 in theexpression of the PRML method.

[0033] In the comparison between FIG. 6B and FIG. 7B, the number of bitarrays and the number of target levels in FIG. 6B do not match withthose in FIG. 7B. If a circuit is configured in accordance with eitherof the methods, only the signals in one method are reproduced; thesignals in the other method cannot be reproduced. However, the presentinvention enables signals of both 2T and 3T run lengths to be reproducedwith one method if the difference between FIG. 6B and FIG. 7B is handledas a non-linear shift, since the present invention can compensate such anon-linear shift. The method can be summarized as follows.

[0034] (1) One of the two methods, which handles signals that areshorter in minimum run length (PLL(1, 7), PR(1, 2, 2, 1)), is selectedas the basic method.

[0035] (2) A compensation value is added to a target value decided bythe convolution of a bit array and each PR class coefficient accordingto the subject bit array and the result is used as a new target value.

[0036] (3) The following conditions are used to reproduce signals thatare shorter in minimum run length (PLL(1,7)).

[0037] (4) When reproducing signals that are longer in minimum runlength (RLL(2,10), PR(3,4,4,3)), the difference between the target valueof the shorter smaller minimum run length (RLL(1,7), PR(1,2,2,1)) andthe target value of the longer minimum run length is used as acompensation value corresponding to the subject bit array. At the sametime, because there are always bit arrays in which a run length limiterror occurs respectively, a compensation value (regardless of the signtype (positive/negative) large enough with respect to the amplitude ofthe reproduced signal is used so as to obtain a difference between thetarget value and the signal, which is large enough to satisfy the runlength limit practically.

[0038] According to the present invention, non-linear componentsincluded in each reproduced signal are suppressed and the S/N ratio isimproved practically, so that the present invention can realize bothsignificant expansion of disk capacity and correspondence to signalshaving different minimum run lengths.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039]FIG. 1 is a table for describing the basic concept of aninformation reproducing method of the present invention;

[0040]FIG. 2A and FIG. 2B are respectively a graph and a table of testresults for denoting a relationship between the recording capacity andthe bit error rate with respect to a (1+D)^(n) PR class series;

[0041]FIG. 3A and FIG. 3B are respectively a graph and a table of testresults for denoting a relationship between the recording capacity andthe bit error rate with respect to a (1, 2, . . . , 2, 1)PR classseries;

[0042]FIG. 4A and FIG. 4B are respectively a graph and a table of testresults for denoting a relationship between the recording capacity andthe bit error rate with respect to an impulse response likelihood PRclass series;

[0043]FIG. 5A and FIG. 5B are respectively a graph and a table of testresults for denoting a difference of capacity expansion capabilitybetween the present invention and the conventional one;

[0044]FIG. 6A and FIG. 6B are tables for denoting a relationship betweenthe bit array and the target value for reproducing RLL (1, 7) modulatedsignals (minimum run length: 2T) with use of the PR class PR(1,2,2,1);

[0045]FIG. 7A and FIG. 7B are tables for denoting a relationship betweenthe bit array and the target value for reproducing RLL (2,10) modulatedsignals (minimum run length: 3T) with use of the PR class PR(3,4,4,3);

[0046]FIG. 8 is a run length limit compensation table for storing setvalues used to reproduce signals with the minimum run length 3T with useof the compensated PR(1,2,2,1) method for the minimum run length 2T ofthe present invention;

[0047]FIG. 9 is a table for storing initial target values, compensationvalues, and compensated target values with respect to the conventionalcompensated PR(1, 2,2,1) method;

[0048]FIG. 10 is a table for storing initial target values, compensationvalues, and compensated target values with respect to the compensatedPR(0,1,2,2,1,0) method of the present invention;

[0049]FIG. 11 is a block diagram of a decoder with respect to the basicmethod of the present invention;

[0050]FIG. 12 is a combined block diagram of the decoder and acompensation table study unit with respect to a study method forextracting the compensation value of the present invention from areproduced signal;

[0051]FIG. 13 is a combined block diagram of the decoder and an RFcompensation unit with respect to a method for obtaining a compensatedreproduced signal of the present invention;

[0052]FIG. 14 is a combined block diagram of the decoder, the RFcompensation unit, and a PLL circuit with respect to a method forobtaining a clock used to reproduce information with use of compensatedreproduced signals of the present invention;

[0053]FIG. 15 is a combined block diagram of the decoder, thecompensation table study unit, and a RLL compensation table with respectto a method for corresponding to signals having different run lengthswith use of the reproducing method of the present invention;

[0054]FIG. 16 is a run length limit compensation table for storing setvalues used to reproduce signals of the minimum run length 3T with useof the minimum run length 2T compensated PR(0,1,2,2,1,0) method of thepresent invention;

[0055]FIG. 17 is a graph of measurement results for denoting arelationship between the tangential tilt and the bit error rate ofDVD-RAM disks according to the present invention; and

[0056]FIG. 18 is a concept chart of an optical disk drive of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0057] Hereunder, the preferred embodiments of the present inventionwill be described with reference to the accompanying drawings.

[0058]FIG. 1 shows the basic concept of an information reproducingmethod of the present invention. In order to simplify the description,the most basic class PR (1,1) will be picked up as the subject here.Method 1 is the basic PRML (conventional) method. As shown in theconfiguration example, each reproduced signal is compared with a targetvalue with respect to the bit arrays for two consecutive times to selecta bit array having the minimized error. In this example, the number oftarget levels is 3 and the method cannot solve the problems of asymmetryoccurrence and non-linear inter-symbol interference of reproducedsignals.

[0059] Method 2 is the PRML method disclosed in the Technical Digest ofISOM 2002 (269-271 (2002)). In this method, a compensation value Vcorresponding to a 2-bit bit array is added to a target value decided bya convolution operation to obtain a new target value, which is then usedto select a bit array that minimizes the error from the value of thereproduced signal while transforming the bit array to a binary one. Thenumber of compensation values V becomes 4(2²). This method can changethe target value suitably corresponding to the asymmetry of thereproduced signals, but cannot remove the non-linear inter-symbolinterference completely.

[0060] In the method of the present invention, a pattern compensationbit is added both before and after each PR(1,1) bit array. Unlike themethod 2, this method is characterized in that a compensation value V isadded to a target value corresponding to a 4-bit bit array to which twopattern compensation bits are added. Then, the target valuecorresponding to the 4-bit bit array is compared with each reproducedsignal to select a bit array in which the error is minimizedsequentially, then transform the selected bit array to a binary one. Inthis method, the number of compensation values V is 16 (=2⁴) while thenumber of target levels is kept at 3, so that the non-linearinter-symbol interference can be compensated within a range of the 4-bitbit array.

[0061] As described above, the present invention realizes significantexpansion of disk capacity as follows.

[0062] (1) The number of class bits is suppressed from increasing so asnot to increase the number of target levels.

[0063] (2) A compensation value corresponding to the subject bit arrayis added to a target value decided by a convolution operation tocompensate the target value, thereby removing non-linear componentsincluded in each reproduced signal.

[0064] Basically, this method that satisfies the above two conditions isalso the basic PRML method. However, the method is characterized in thata target value is decided with two bits. This method can thus removenon-linear components without increasing the number of target levels.

[0065] To distinguish this method from the conventional PRML method, thePR class expression will be described as compensated PR(0,1,1,0). Inthis method, the number of class bits is 4 just like in the PRML methodand each target value is calculated by a convolution operation of acoefficient array (0,1,1,0) and a 4-bit bit array just like in theconventional description. However, the coefficient of each of the bitsat both ends is zero, so that it becomes the same as the target valuedecided with a 2-bit coefficient array (1,1). The coefficient “0” placedat both ends denotes pattern compensation bits and the compensated PRmeans addition of a compensation value V decided by a 4-bit bit array toa target value. Similarly, the conventional method 1 can be expressed asPR(1,1) while the method 2 can be described as compensated PR(1,1).

[0066] The test results shown in FIG. 6 are obtained when signals havingdifferent recording density values are recorded on an optical disk andreproduced with each of the methods described above. The test conditionsare as follows. The optical disk used in the test is formed bylaminating phase-change films on a substrate structured so as to havelands and grooves at track pitches of 0.34 μm. A DDU-1000 type opticaldisk evaluator manufactured by PulseTech Products Corporation was usedfor the test. The wavelength of the light source was 405 nm and theobjective lens NA was 0.85. The RLL(1, 7) was used as the modulationcode and the detection window width Tw was changed within a range of 53to 80 nm. The recording capacity of one side of a CD-size disk wasassumed as 35 GB at Tw=53 nm. The test results shown in FIG. 6 wereobtained at Tw=57 nm and basic PR class=PR(1,2,2,1) when the recordingcapacity was 32.5 GB and information was reproduced from the opticaldisk at a data transfer rate of 100 Mbps. The bit error rate was 50×10⁻⁴in the method 1 (PR(1,2,2,1)), 15×10⁻⁴ in the method 2 (compensatedPR(1,2,2,1)), and 0.05×10⁻⁴ in this method (compensated PR(0,1,2,2,1,0).It was thus assured that this method could suppress the bit error rateat {fraction (1/100)} and under. In addition, the reproduced signaleye-pattern denotes that of each practical signal (compensatedreproduced signal) when each of the above methods is used. In thismethod, it is confirmed that the eye is opened clearly. The S/N ratio ofeach 2Tw signal included in each compensated reproduced signal was 3.6dB in the method 1, 6.1 dB in the method 2, and 9.5 dB in this method.The details of the compensated reproduced signal will be describedlater.

[0067]FIG. 5 shows test results of the difference of capacity expansionperformance between the method of the present invention and theconventional method. The PR(1,2,2,1) was selected as the basic PR classin the test. The allowable value of the bit error rate was assumed as10⁻⁴ to obtain the upper limit of the recording capacity. The recordingcapacity upper limit of the conventional method was 30 GB at PR(1,2,2,1)and 32 GB at compensated PR(1,2,2,1). The recording capacity upper limitof the present invention method was 32.5 GB at compensatedPR(0,1,2,2,1,0)ML4, 34.5 GB at compensated PR(0,1,2,2,1,0), and 35 GBand over at compensated PR(0,0,1,2,2,1,0,0) and compensatedPR(0,0,0,1,2,2,1,0,0,0) respectively. The compensated PR(0,1,2,2,1,0)ML4 denotes a method in which 6 bits were used to decide only acompensation value while the number of bits used for the most likelihoodcomposite processing to select the most likelihood bit array was kept at4 bits. This method obtained better test results than the conventionalmethod. However, because the most likelihood composite processing wasdone without including any pattern compensation bit in a bit array, thenon-linear shift could not be suppressed satisfactorily. This is why themost likelihood composite processing in which pattern compensation bitsare included becomes very important to bring out the capability of thePRML method of the present invention. The test results obtained here areeffective to increase the recording capacity more than the case in whichthe basic PR class is PR(1,2,2,1) and other cases of the various PRclasses that use the conventional PRML methods shown in FIG. 2A throughFIG. 4.

[0068]FIG. 5B shows the number of bit arrays, the number of states, thenumber of levels, the number of pattern compensation bits, and thenumber of ML bits with respect to each of the methods shown in FIG. 5A.The scale of a circuit for employing the PRML method is roughlyproportional to the number of bit arrays, so that the circuit scale mustbe expanded more than 10 times that of the PR(1,2,2,1) method to realizea compensated PR(0,0,01,2,2,1,0,0,0) class in which 3 patterncompensated bits are added to each of the ends. Because the presentinvention can reproduce signals having different run lengths, thecircuit scale, if it is three times or so and can improve theperformance, could be considered to be proper, since both PR(1,2,2,1)and PR(3,4,4,3) classes have been required to be mounted conventionallyto enable the compatibility among reproduced signals having differentminimum run lengths. Considering those various requirements describedabove, the compensated PR(1,2,2,1,0) class in which two patterncompensated bits are added might be the best selection practically.

[0069] While a description has been made so far for a method thatselects PR(1,2,2,1) as the basic PR class and adds the same number ofpattern compensation bits to both before and after each bit array, thepresent invention is not limited only to that; any other basic class,for example, PR(1,1), PR(1,2,1), PR(3,4,4,3), PR(1,1,1,1),PR(1,2,2,2,1), etc. may be selected. The number of pattern compensationbits may not be the same before and after each bit array; they may beasymmetrical, such as compensated PR(0,1, 2,2,1), PR(0,0,1,2,2,1),compensated PR(1,2,2,1,0), compensated PR(1,2,2,1,0,0), etc. Forexample, if a signal in which it is clear that the recording timethermal interference is concentrated physically at the front edge is tobe reproduced, it would be best to add a pattern compensation bit onlyat the front side sometimes.

[0070]FIGS. 9 and 10 show compensation values V used for the testperformance shown in FIG. 1. FIG. 9 shows initial target values withrespect to the conventional compensated PR(1,2,2,1) method (obtained byeach convolution operation of a bit array and a PR class coefficientarray), compensated values corresponding to bits arrays, and compensatedtarget values, each obtained by adding a compensation value to aninitial target value. 0.180 is added to a bit array “0000” correspondingto a long space while 0.090 is added to a bit array “1111” correspondingto a long mark. As to be understood clearly from the eye pattern of themethod 1 of the test results shown in FIG. 1, it denotes a resultcorresponding to an asymmetrical pattern in which a 2T signal comeslower than the center of the signal amplitude.

[0071]FIG. 10 shows a table of initial target values, compensationvalues, and compensation target values with respect to the compensatedPR(0,1,2,2,1,0) class of the present invention. In this table, thecompensation value of the bit array No.3“000110” is 0.129 while thecompensation value of the bit array No.4“000111” is −0.056. The formercorresponds to a 2Tw mark while the latter corresponds to a mark of 3Twand over. This compensation value difference is caused by adaptation ofthe compensated PR(0,1,2,2,1,0) class to a non-linear shift between a2Tw mark and a mark of 3Tw and over. The conventional PRML method, whichhas no pattern compensation bits, cannot correspond to the non-linearshift like that.

[0072]FIGS. 11 through 15 show a block diagram for realizing each PRMLmethod block of the optical disk unit that employs the informationreproducing method of the present invention respectively.

[0073]FIG. 11 shows a block diagram of the basic method of the presentinvention. The decoder 10 is configured by a waveform equalizer 11, abranch metric calculation unit 12, an ACS (ADD Select Compare) unit 13,a path memory 14, a PR target table 15, and a pattern compensation table16. An RF input (reproduced signal) 50 that is already converted to adigital value in an AD converter is subjected to an equalizationprocessing through a FIR filter in the waveform equalizer 11, then asquared value (branch metric value) of an error from a target value iscalculated for each bit array in the branch metric calculation unit 12.The target value at that time is obtained by adding up an initial targetvalue and a compensation value obtained with reference to the PR targettable 15 and the pattern compensation table 16 corresponding to the bitarray.

[0074] The ACS unit 13 adds a branch metric value corresponding to eachbit array to a metric value in the previous time state and each state (abranch metric value is added sequentially upon a state change, thenprocessed so as not to be diffused). At this time, a state having asmaller metric value is selected from those in the transition step up tothe current time state (usually, two states, but only one statesometimes due to the run length limitation). A “state” means a bit arraystored with respect to a time state change. For example, when the numberof PR class bits is 4, the bit array is expressed by 4 bits and thestate is expressed by 3 bits.

[0075] The path memory 14 stores a binary value compounded for each bitarray for a long time. In the memory, the data is shifted each time thetime is updated, so that the memory always comes to store the latestdata. The ACS unit 13 alters the arrangement order of the informationstored in the path memory according to the selected transition processeach time a transition process is selected. By repeating the aboveprocessings in the above units as described above, information in thepath memory is integrated step by step. After a long time, the samevalue comes to be stored for each bit array, that is, path merging iscompleted. The transformed binary value 51 means binary informationtaken out from the path memory when the time is updated.

[0076] While a PR class target value is obtained with reference to thePR target table 15 in the above description, such a target value hasoften been calculated directly with use of a product sum computing unit.The main point of the present invention is to calculate a new targetvalue by adding a compensation value to the initial target value, thentransforming the result to a most likelihood binary value. Thus, both ofthe methods may be used to calculate such a target value with noproblem.

[0077]FIG. 12 shows a block diagram for realizing a study method forextracting the compensation value of the present invention from areproduced signal. Both configuration and operation of the decoder 10are as described above. Here, only the study method will be described.

[0078] The compensation table study unit 20 is configured by a targetcalculation unit 21 and an error calculation & averaging unit 22. Theunit 21 stores the binarized data 51 for the number of class bits andadds up an initial target value and a compensation value according tothe bit array obtained with reference to the PR target table 15 and thepattern compensation table 16, then the result is used to calculate anew target value. The processing contents are basically the same asthose of the branch metric calculation unit 12. The branch metriccalculation unit 12, however, calculates values for the number of bitarrays simultaneously while the target calculation unit 21 uses only thebinarized data to calculate only one target value.

[0079] The error calculation & averaging unit 22 calculates an errorvalue between the outputs of the waveform equalizer 11 and the targetcalculation unit 21 by comparing one output with the other. The errorvalues are classified by bit arrays and error values obtained within aselected time are averaged to obtain an error table value correspondingto the object bit array. When such an error table value is calculated,the output of the waveform equalizer 11 is delayed by the value of ashift register, then adjusted to the delay from the output of the targetcalculation unit 21. The error table value 52 obtained such way is addedto each value in the pattern compensation table 16, thereby eachcompensation value can follow up with the reproduced signal properly.

[0080] It would be natural to begin each initial value with zero in thepattern compensation table 16. And, it would be more efficient to useeach initial value prepared for each medium to make good use of thestudy results. Such an initial value is obtained from simulation, actualmeasurement, the last used value with respect to a similar disk loadedin the drive.

[0081]FIG. 13 shows a block diagram for realizing the compensatedreproduced signal obtaining method of the present invention. Thecompensated reproduced signal shown in FIG. 1 can be decided for itsquality intuitively. An actual waveform equalization signal has the sameeye-pattern as that shown in the method 1 in FIG. 1. If an optical diskis expanded significantly in capacity, most of 2T signals cannot bedecomposed. Thus, the signal quality cannot be evaluated intuitively ata glance of this eye-pattern, so that it is not decided whether or notan error has occurred in the disk. For an optical disk, because the diskmust be loaded and operated in drives of a plurality of makers, therecording/reproducing compatibility among those makers is considered asthe most important item in the development. This is why evaluation isdone for the difference of performance and the characteristicperformance of each disk medium even among large capacity optical diskdrives. In this connection, the bit error rate and the eye-pattern haveconventionally been checked visually to evaluate the signal quality.Consequently, the eye-pattern that makes it possible to check the signalquality intuitively and visually will become indispensable even forlarge capacity optical disks in which the minimum run length signal canhardly be decomposed.

[0082] The configuration and operation of the decoder 10 in FIG. 13 areas described above. Here, a description will be made for how to obtain acompensated reproduced signal. The compensated reproduced signal 53 ofthe present invention is output from the reproduced signal compensationunit 30. The unit 30 is configured by a compensation computing unit 31and a D/A converter 32. The computing unit 31 stores binarized data 51for the number of class bits and obtains a compensation value from thepattern compensation table 16 according to each bit array, thensubtracts the compensation value from the output of the waveformequalizer 11. At this time, just like the error calculation in thecompensation table study unit 20 and the operation of the errorcalculation & averaging unit 22, the computing unit 31 adjusts the valueof the delay from the waveform equalizer 11. The digital data outputfrom the compensation computing unit 31 is converted to an analog signalby the D/A converter 32 to obtain the compensated reproduced signal 53.

[0083] The compensated reproduced signal 53, from which non-linear shiftcomponents are eliminated beforehand, can be transformed to a binaryvalue with use of the conventional PRML method. The binary valueobtained is the same as the binary value obtained with the compensationPRML method of the present invention. From this point of view, thereproduced signal compensation unit 30 can be taken as an excellentnon-linear equalizer. Each of general waveform equalizers including that11 shown here is composed as an FIR (Finite Impulse Response) filter, sothat an equalized waveform is obtained by a sum product operation of asignal and a coefficient array. This is why the characteristic of awaveform equalizer is often defined as a frequency characteristic. AnFIR filter returns linear type responses. If a waveform equalizer thatamplifies 2T signals, for example, the equalizer also comes to amplifythe noise in the 2T signal band together.

[0084] On the other hand, in the processing executed in the reproducedsignal compensation unit 30, no sum product operation is made tosubtract a compensation value. The frequency characteristic thus becomesnon-linear, so that 2T signals can be amplified without amplifying thenoise in the same signal band. The S/N ratio of the compensatedreproduced signal 53 can be set larger than that of any other waveformequalizer that uses an FIR filter. The difference of the S/N ratio amongvarious methods shown in FIG. 1 is brought with this effect. And, thepresent invention can improve the S/N ratio of the reproduced signalspractically, thereby realizing significant expansion of optical diskcapacity.

[0085]FIG. 14 shows a block diagram for realizing a method for obtaininga clock used to reproduce information with use of the compensatedreproduced signal of the present invention. The configurations andoperations of the decoder 10 and the reproduced signal compensation unit30 shown in FIG. 14 are already described above. A description will thusbe made here only for how to generate a clock.

[0086] To take out information from reproduced signals and binarize theinformation, a reference clock is needed. Such a clock, as well known,is often generated by a PLL (Phase Locked Loop) circuit. If therecording density increases until the minimum run length signal is notdecomposed any more and reproduced signals are connected to a PLLcircuit directly to generate a clock, the generated clock often becomesunstable, since the S/N ratio of the minimum run length signal is small.The shift included in each reproduced signal also becomes an externaldisturbance factor for joggling the clock, causing the clock to becomeunstable. As shown above, the compensation signal of the presentinvention can improve the S/M ratio and compensate the non-linear shift.If the compensated reproduced signal is connected to a PLL circuit togenerate a clock, therefore, the clock will become stable very much. Asshown in FIG. 13, the compensated reproduced signal 53 is just requiredto be connected to the PLL circuit 18 to generate the block 54 in thisconnection.

[0087] As described above, if a clock is generated from a compensatedreproduced signal, the S/N ratio of minimum run length signals becomeslow. To solve such a problem, the PLL circuit may be provided with meansfor removing minimum run length signals to generate a clock. And,because the method can transform signals to binary values, the methodmay be used to remove minimum run length edge information from eachphase error signal to be sent to the VCO (Voltage Controlled Oscillator)provided in the PLL circuit. In addition, because the use of a digitalPLL circuit is widespread in recent years, the above method can berealized only by adding a simple-structured logic circuit to the PLLcircuit. The test results shown in FIGS. 1 through 5 are obtained byremoving minimum run length signals to operate the PLL circuit. Thegenerated clock 54 may be supplied to the decoder 10, etc. The internalconfiguration of the PLL circuit is well known, so the description willbe omitted here.

[0088]FIG. 15 shows a block diagram for realizing an informationreproducing method of the present invention to process signals havingdifferent run lengths. The configuration shown in FIG. 15 is obtained byadding a run length limit compensation table 40 to the configurationshown in FIG. 12. The basic operation of the configuration is the samebetween FIG. 12 and FIG. 15 and it is already described above.Therefore, only the functions of the run length limit compensation table40 will be described here. The summarization of the basic concept of theconfiguration will be as follows.

[0089] (1) A method that uses smaller minimum run length signals isselected as the basic method.

[0090] (2) A new target value is obtained by adding a compensation valuecorresponding to a bit array to the initial target value decided by aconvolution operation of a bit array and a PR class coefficient.

[0091] (3) Shorter run length signals are reproduced on the aboveconditions.

[0092] (4) Each of longer run length signals is reproduced using acompensation value according to each bit array. The compensation valueis a difference between the target value of shorter run length signalsand the target value of longer run length signals. Values (regardless ofthe sign) large enough with respect to the amplitude of reproducedsignals are used as those compensation values to assume a largedifference between a target value and each signal so as to limit the runlength practically.

[0093] The run length limit compensation table 40 shown in FIG. 15 maybe any one if it stores compensation values generated in accordance withthe concept described in (4). The initial values of the patterncompensation table 16 may be the values of the run length compensationtable 40 first when in reproducing of signals. The operations after thatare the same as those described above.

[0094] In this connection, note the following one point, that is, thecompensation value for each bit array that causes a run length limiterror therein. To reproduce extremely low quality signals such as thosereproduced from a medium on which dust is stuck, a defective medium, orthe like, even this method cannot avoid reproduction error occurrence.Such a reproduction error causes incomplete path merging. As a result,the binary value 51 comes to include a bit array in which a run lengtherror has occurred. In this connection, if the above-described studyprocessing is executed for the bit array, the compensation value of therun length error-occurred bit array is updated. If such an event isrepeated, the practical run length error evasive function will be lost.And, to solve such a problem, the compensation value corresponding to abit array that causes a run length error should keep a value largeenough (any sign) with respect to the amplitude of the reproducedsignals. Concretely, when a study result 53 is stored in the patterncompensation table 16, the compensation value of the subject bit arrayis limited so as not to be updated or only the compensation value may beoverwritten with a value stored in the run length limit compensationtable 40 after it is updated.

[0095]FIG. 8 shows concrete values set in the run length limitcompensation table. The values set in this table are those of thesignals having the minimum run length of 2T with the compensated PR(1,2,2,1) of the basic method. If this basic method is used to reproducesignals having the minimum run length of 3T with the compensated PR(3,4,4,3), PLL compensation values that can eliminate the difference amongtarget values corresponding to each bit array may be set in the runlength limit compensation table. For example, 0.000 may be used for thebit array “0000” and −0.095 may be used for the bit array “0001”. And,because a run length limit error occurs in both of the bit arrays “0110”and “1001”, a large value should be set in them respectively.Concretely, the value should be more than double the reproduced signalamplitude (2 in this example). Here, the value will be described as ∞.

[0096]FIG. 16 shows another concrete example of values set in the runlength limit compensation table. In this case, the compensatedPR(0,1,2,2,1,0) is selected basically. The description for the table isthe same as that in FIG. 15. No special description will thus be madehere.

[0097] To quantify the effect for reproducing signals having differentrun lengths, a 4.7 GB DVD-RAM disk available on the market was used inthe test to reproduce information therefrom by employing the compensatedPR (1,2,2,1) corresponding to the minimum run length 2T basically.

[0098]FIG. 17 shows measurement results of a relationship between thetangential tilt and the bit error rate of the sample DVD-RAM disk. Asshown in FIG. 17, when the conventional PR(1,2,2,1) method was used toreproduce the information from the DVD-RAM disk, the bit error ratenever went under 10⁻⁴. On the other hand, when the compensatedPR(1,2,2,1) method was used basically and the values shown in FIG. 8were selected as the initial values, the bit error rate became 10⁻⁴ andunder within ±0.5° or more. The margin value was wider than that of theconventional PR(3,4,4,3) method configured for the DVD-RAM disks and theeffect of the present invention was proved.

[0099] In FIGS. 11 through 15, both configuration and operation of thePRML method of the present invention have been described. When anoptical disk drive or magnetic disk drive is used actually, however, thedrive will need still many peripheral parts that are not shown in FIGS.11 through 15. For example, the drive will need a high-path filter forsuppressing DC components of reproduction head signals, a pre-equalizerand a low-path filter for improving frequency characteristics, an autogain control circuit for fixing amplitudes at a value, an A/D converterfor converting analog signals to digital signals, etc. that must beadded before each of the basic block configurations shown in FIGS. 11through 15 is formed. There are also other necessary components such asa PLL circuit for generating a clock from the output of the waveformequalizer, a fault preventive function for holding the operations of thePLL circuit, the AGC circuit, etc. by detecting faults, head crushes,etc. Those functions are known widely and to be mounted easily by thosewho are skilled in the art. Thus, no special description will be madehere.

[0100]FIG. 18 shows a block diagram of an optical disk drive of thepresent invention. An optical disk 100 is rotated by a spindle motor162. An optical head 130 is configured by a light source 131, anobjective lens 132, and a detector 133. The optical head 130 ispositioned at a given position in the radial direction of the opticaldisk 100 by automatic control means 161 provided in servo mechanismcontrol means 160. Light power control means 171 controls the lightsource 131 to emit a light 122 having an intensity specified from a CPU151. The light 122 is condensed by the objective lens 132 to form a spot101 on the optical disk 100. The objective lens 132 makes both focusingand tracking operations under the control of the automatic positioncontrol means 161. A reflection light 123 from the spot 101 is convertedto electrical signals to become reproduced signals in the detector 133.

[0101] The reproducing means 190 uses reproduced signals to reproducecode and address information recorded on the optical disk 100. Thefunctions that realize the information reproducing method of the presentinvention described above are all stored in the reproducing means 190.Those functions may also be executed by an LSI in which the unitsconfigured as shown in the block diagrams in FIGS. 11 through 15 aremounted.

[0102] Hereunder, other aspects of the present invention will bedescribed.

[0103] (1) According to one of the aspects of the present invention, theinformation reproducing method uses a PRML method in which a set ofclass bits is combined with a compensation bit used to distinguishedbetween a mark and a space, thereby compensating the non-linear shift ofeach signal waveform to reproduce information from an optical disk.

[0104] (2) According to another aspect of the present invention, theinformation reproducing method uses the PRML method for binarizing eachreproduced signal by selecting the most likelihood state change in acomparison between a target signal and the reproduced signal for acontinuous N time.

[0105] The method, if the PRML method is expressed as PR(α₁, α₂, . . . ,α_(N))ML, comprises:

[0106] a step of using a target signal obtained by adding 2N or lesscompensation values V2 corresponding to the value of an N-bit digitalbit array to the initial target level V1 obtained by a convolutionoperation of N coefficient values (α₁, α₂, . . . , α_(N)) and an N-bitdigital bit array;

[0107] a step of binarizing the reproduced signal to the most likelihoodbit array while comparing the reproduced signal with the target value(V1+V2); and

[0108] a step of obtaining a compensated reproduced signal bycalculating a compensation value V2 for each group of N bits in abinarized bit array, then subtracting the result from the reproducedsignal.

[0109] (3) According to still another aspect of the present invention,the information reproducing drive for outputting a binarized value fromeach reproduced signal with use of the PRML method comprises:

[0110] a state change logic corresponding to the minimum run lengthR1(R1≧1);

[0111] a PR target value output unit for outputting a PR class targetvalue corresponding to an N-bit bit array;

[0112] a compensation table for storing a pattern compensation valuecorresponding to each M-bit (M≧N) bit array;

[0113] a waveform equalizer for equalizing the reproduced signal; and

[0114] a branch metric calculation unit for calculating a branch metricvalue for each bit array using a target value obtained by adding a PRtarget value output from the PR target value output unit and acompensation value stored in the pattern compensation table to theoutput of the waveform equalizer.

[0115] When the information reproducing drive is to reproduce a runlength limit signal having the minimum run length of R2 (R2>R1), thedrive sets a value larger or smaller enough than the maximum or minimumoutput value of the PR target value output unit for the patterncompensation value selected from the pattern compensation values in thepattern compensation table corresponding to a bit array whose run lengthis at least R2 and under, thereby the reproduced signal is binarizedcorresponding to the minimum run length R2 practically.

[0116] As described above, the information reproducing method and theoptical disk drive of the present invention can correspond tosignificant expansion of disk capacity and process signals havingdifferent minimum run lengths.

What is claimed is:
 1. An information reproducing method that employs aPRML method for comparing a target signal with each reproduced signalfor a continuous N time to select the most likelihood one of statechanges therein, thereby transforming said reproduced signal to a binaryvalue; wherein, when said PRML method is represented as PR(α1, α2, . . ., αN), the leftmost M1 coefficient and the rightmost M2 coefficient in acoefficient array are all zero while integer values M1 and M2 satisfy arelationship of “M1≧0, M2≧0, M1+M2≧1, M1+M2<N”; and wherein, wheninteger MM=M1+M2 and integer NN=N−MM are satisfied, said methodincludes: a step of using a target value obtained by adding 2^(N) orless compensated values V2 corresponding to a value of an N-bit digitalbit array to an initial target level V1 obtained by a convolutionoperation of each of NN non-zero coefficient values and an NN-bitdigital bit array; and a step of binarizing said reproduced signal tothe most likelihood bit array while comparing said reproduced signalwith said target value (V1+V2).
 2. An information reproducing methodthat employs a PRML method for selecting the most likelihood one ofstate changes in a reproduced signal while comparing a target signalwith each reproduced signal obtained for a continuous N time, therebybinarizing said reproduced signal to a binary value; wherein, when saidPRML method is represented as PR(α1, α2, . . . , αN)ML, the logic ofsaid state change excludes a state change logic of a reproduced signalwhose minimum run length is R1 and under in accordance with the minimumrun length R1 (R1≧1); wherein said method includes: a step of using atarget value obtained by adding 2^(N) or less compensation values V2corresponding to a value of an N-bit digital bit array to an initialtarget level V1 obtained by a convolution operation of each of Ncoefficient values (α1, α2, . . . , αN) and an N-bit digital bit array;a step of binarizing said reproduced signal to the most likelihood bitarray while comparing said reproduced signal with said target value(V1+V2); and a step of setting a value larger enough than an amplitudeof said reproduced signal as said compensation value V2 corresponding tosaid digital bit array if the run length of said N-bit digital bit arrayis R2 and under when the minimum run length R2 (R2>R1) signal is to bereproduced.
 3. The method according to claim 1, wherein said methodfurther includes a step of obtaining a compensated reproduced signal bycalculating a compensation value V2 for each group of N bits in saidbinarized bit array, then subtracting the result from said reproducedsignal.
 4. The method according to claim 3, wherein a clock used toreproduce information is extracted from said compensated reproducedsignal.
 5. The method according to claim 1, wherein a clock used toreproduce information is generated without using phase informationobtained from a minimum run length mark.
 6. The method according toclaim 2, wherein a clock used to reproduce information is generatedwithout using phase information obtained from a minimum run length mark.7. An information reproducing drive for outputting a binary valueobtained from a reproduced signal with use of a PRML method, said drivecomprising: a PR target output unit for outputting a PR class targetvalue corresponding to an N-bit bit array; a pattern compensation tablefor storing a compensation value corresponding to each M-bit (M>N) bitarray; a waveform equalizer for equalizing a reproduced signal; and abranch metric calculation unit for calculating a branch metric value foreach bit array by employing a target value obtained by adding up a PRtarget value output from said PR target value output unit and acompensation value stored in said pattern compensation table withrespect to an output from said waveform equalizer.
 8. The driveaccording to claim 7, wherein said drive further includes a compensationtable study unit for correcting said pattern compensation table so as tominimize an error between an output from said waveform equalizer andsaid target value calculated in accordance with an obtained binary bitarray.
 9. The drive according to claim 7, wherein said drive furtherincludes: a compensation calculation unit for storing binary class bitsand obtaining a compensation value corresponding to said bit array, thensubtracting the result from said waveform equalizer; and a D/A converterfor converting an output of said compensation calculation unit to ananalog signal.
 10. The drive according to claim 9, wherein said drivefurther includes a PLL circuit for inputting an output of said D/Aconverter; and wherein said PLL circuit generates a clock.